In the proposed research Phycomyces will be studied as a model system for sensory transduction processes in primary receptor cells. Double mutant strains will be constructed with defects in all pairwise combinations of the seven genes associated with the light responses of the sporangiophore. These strains will first be characterized with respect to the major stimulus-response modes. Then the light growth response will be studied in detail using the white noise method of nonlinear system identification, followed by formal system analysis. Experiments will be performed on the Phycomyces tracking machine. Transfer functions computed from the responses for double-mutant, single-mutant and wild-type strains will be combined mathematically, in order to determine the dynamics and organization of the underlying stimulus-response pathway. In addition mutants with new types of defects in this sensory pathway will be sought, particularly those which indicate alterations in the presumed flavoprotein photoreceptor. These mutants will be classified and then incorporated in the system analysis program described above. A spectrophotometer for measuring light-induced absorbance changes will be modified to permit repetitive flash photometry on a submillisecond time scale. Such absorbance changes will be sought differentially between mutants and wild-type to facilitate the biochemical isolation of the blue-light photoreceptor.